Method of processing silicon wafer

ABSTRACT

The inventive method for processing a silicon wafer is a method comprising step  11  in which a single crystal ingot is sliced into thin disc-like wafers; step  13  in which the surface of each wafer is lapped to be planar; step  14  in which the wafer is subjected to alkaline cleaning to be removed of contaminants resulting from preceding machining; and step  16  in which the wafer is alternately transferred between two groups of etching tanks one of which contain acidic etching solutions and the other alkaline etching solutions, wherein an additional step  12  is introduced between step  11  and step  13  in which a wafer is immersed in an acidic solution containing hydrofluoric acid (HF) and nitric acid (HNO 3 ) at a volume ratio of ⅛ to ½ (HF/HNO 3 ) so that degraded superficial layers occurring on the front and rear surfaces of the wafer as a result of machining can be removed and the edge surface of the wafer can be beveled. 
     The inventive method simplifies the steps involved in the processing of a wafer, and reduces the intervention of alkaline cleaning accompanied with mechanical beveling, thereby reducing the risk of contamination due to metal impurities which may result from alkaline cleaning.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims International Application PCT/JP2004/007251filed May 27, 2004 and priority of Japanese Application No. 2003-150261filed May 28, 2003, the entire disclosure of which is herebyincorporated by reference.

TECHNICAL FIELD

The present invention relates to a method for processing a silicon waferwhich can prevent contamination due to metal impurities which wouldotherwise occur as a result of the cleaning of the wafer performedsubsequent to its having undergone various machining processes.

BACKGROUND ART

Following steps have been disclosed as a conventional method for theprocessing of a silicon wafer (see, for example, Nonpatent literature1).

A single silicon crystal rod pulled up from a crucible is cut at itsfrond and rear ends, and then divided into a number of blocks. The blockhas its periphery abraded until it has a diameter close to a desiredone. Then, an orientation flat or a notch is formed to mark a specifiedorientation of the crystal. The single crystal ingot thus formed isfirst sliced at a specified angle to the longitudinal axis of the ingotwith the blade of a cutter into thin, disc-like slices, therebyproducing a plurality of silicon wafers as shown in FIG. 2( a) (step 1).FIG. 2( b) shows the cross-sections of silicon wafers having undergonethe corresponding steps of FIG. 2( a).

The silicon wafer thus obtained is subjected to cleaning using, forexample, an alkaline detergent in order to remove oils adhered possiblyas a result of processing such as peripheral abrasion, slicing step 1,etc. (step 2)

Then, wafers are automatically transported to and from, for example, bya robot while they are subjected to machining processes and areprocessed into devices. The peripheral edge of a wafer is beveledbecause a wafer whose peripheral edge is not beveled and thus isrectangular will tend to develop notches and flaws, and siliconparticles as a result of mechanical impacts to the wafer encountered byaccident during transportation might degrade the surface of adjacentwafers. To prevent the development of such notches and flaws, theperipheral edge of a silicon wafer is beveled with a diamond grinder(step 3). The beveling performed during step 3 is also effective forpreventing the occurrence of an abnormal development (crown phenomenon)of a film along the periphery of a wafer during the epitaxial growth ofa film in a subsequent step or during the application of a photo resist.A wafer having undergone step 3 or beveling step is subjected toalkaline cleaning (step 4). During step 4, contaminants that might occuras a result of beveling during step 3 are removed.

Then, degraded superficial layers developed on the surfaces of a waferwhich would occur as a result of slicing performed during step 1 areremoved by lapping so that the smoothness of the surfaces of the waferand their parallelism can be improved (step 5). After the lapping step5, the wafer is subjected to alkaline cleaning (step 6). During step 6,contaminants that might occur as a result of lapping during step 5 areremoved.

Then, the wafer having undergone the lapping step 5 is subjected to finebeveling (step 7). The wafer having undergone the fine beveling step 7is subjected to alkaline cleaning (step 8). During step 8, contaminantsthat might occur as a result of fine beveling during step 7 are removed.

Subsequently, the wafer is etched by being immersed in an acidic etchingsolution (step 9). During step 9, any degraded superficial layers thatmight occur on the front and rear surfaces of the wafer as a result ofthe above machining are completely eliminated.

Wafers processed as described above will undergo additional processing(not illustrated) such as abrasion, heating, mirror polishing, cleaning,etc. to receive further processing necessary for the fabrication ofdevices.

Nonpatent literature 1: Shimura, F., “Engineering of SemiconductorSilicon Crystals,” Maruzen, Sep. 30, 1993, p. 104.

DISCLOSURE OF INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

With a conventional process as described in the nonpatent literature 1,each time a wafer has been subjected to machining such as beveling orfine beveling like step 3 or 7 described above, an alkaline cleaningstep must be introduced without fail to remove contaminants occurring asa result of the machining. This possesses another problem that thedetergent used for alkaline cleaning itself contaminated withimpurities, particularly metal impurities.

An object of the present invention is to provide a method for processinga silicon wafer enabling the simplified processing thereof.

Another object of the present invention is to provide a method forprocessing a silicon wafer which can reduce the risk of a wafer beingcontaminated by metal impurities subsequent to alkaline cleaning byminimizing the intervention of alkaline cleaning which would beotherwise introduced each time subsequent to beveling.

Means for Solving the Problems

An aspect of the invention represented by claim 1 relates to an improvedmethod for processing a silicon wafer. As shown in FIG. 1( a), themethod comprises a slicing step 11 in which a single crystal ingot issliced into thin disc-like wafers, a lapping step 13 in which thesurface of each wafer is lapped to be planar, an alkaline cleaning step14 in which the wafer is subjected to alkaline cleaning to be removed ofcontaminants resulting from preceding machining, and an etching step 16in which the wafer is alternately transferred between two groups ofetching tanks one of which contain acidic etching solutions and theother alkaline etching solutions.

The method is characterized by its including a beveling step 12 betweenthe slicing step 11 and the lapping step 13 in which a wafer is immersedin an acidic solution containing hydrofluoric acid (HF) and nitric acid(HNO₃) at a volume ratio of ⅛ to ½ (HF/HNO₃) so that degradedsuperficial layers occurring on the front and rear surfaces of the waferas a result of machining can be removed and its edge surface can bebeveled.

According to the aspect of the invention represented by claim 1, insteadof mechanical beveling employed by the conventional method, step 12 isintroduced in which a wafer is immersed in an acidic solution containinghydrofluoric acid (HF) and nitric acid (HNO₃) at a volume ratio of ⅛ to½ (HF/HNO₃) which enables simultaneously the removal of degradedsuperficial layers resulting from preceding machining as well asbeveling of the edge of the wafer. Thus, the method simplifiesprocessing of the wafer. The method further dispenses with the necessityof alkaline cleaning which must be introduced subsequent to mechanicalmachining, which will reduce the risk of the wafer being contaminatedwith metal impurities, particularly Cu-based impurities associated withalkaline cleaning.

Another aspect of the present invention represented by claim 2 relatesto a modification of the method of claim 1 in which etching using anacidic solution containing hydrofluoric acid and nitric acid is adjustedsuch that the total etching rate for both surfaces of a wafer is in therange of 0.1 to 1.0 μm/sec.

Yet another aspect of the invention represented by claim 3 is anothermodification of the method of claim 1 in which the etching step 16includes acidic etching 16 a followed by alkaline etching 16 b, thealkaline etching solution has a concentration of 8 mol/l or higher, andthe total etching rate for both surfaces of a wafer during acidicetching is adjusted to be 0.2 μm/sec or higher.

EFFECT OF THE INVENTION

As described above, the inventive method relates to an improved methodfor processing a silicon wafer, the method comprising a slicing step inwhich a single crystal ingot is sliced into thin disc-like wafers, alapping step in which the surface of each wafer is lapped to be planar,an alkaline cleaning step in which the wafer is subjected to alkalinecleaning to be removed of contaminants resulting from precedingmachining, and an etching step in which the wafer is alternatelytransferred between two groups of etching tanks one of which containacidic etching solutions and the other alkaline etching solutions. Thefeature of the inventive method lies in its including a beveling stepbetween the slicing step and the lapping step in which a wafer isimmersed in an acidic solution containing hydrofluoric acid (HF) andnitric acid (HNO₃) at a volume ratio of ⅛ to ½ (HF/HNO₃) so thatdegraded superficial layers occurring on the front and rear surfaces ofthe wafer as a result of machining can be removed and its edge surfacecan be beveled.

According to the inventive method described above, instead of mechanicalbeveling employed by the conventional method, a step is introduced inwhich a wafer is immersed in an acidic solution containing hydrofluoricacid (HF) and nitric acid (HNO₃) at a volume ratio of ⅛ to ½ (HF/HNO₃)which enables simultaneously the removal of degraded superficial layersresulting from preceding machining as well as beveling of the edge ofthe wafer. Thus, the method simplifies processing of the wafer. Themethod further dispenses with the necessity of alkaline cleaning whichmust be introduced subsequent to mechanical machining, which will reducethe risk of the wafer being contaminated with metal impurities,particularly Cu-based impurities associated with alkaline cleaning.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiments of the invention will be described below with referenceto attached drawings.

First, a grown single silicon crystal rod is cut at its frond and rearends, and then divided into a number of blocks. The block has itsperiphery abraded until it becomes a cylinder having a uniform diameter.Then, an orientation flat or a notch is formed to mark the specifiedorientation of the crystal to produce a single crystal ingot product.The orientation flat and the notch are utilized as a reference forplacing the wafer properly with respect to masks in a subsequent stepduring its elaboration into a device. After the step, the single crystalingot is sliced at a specified angle to the longitudinal axis of theingot with the blade of a cutter into thin, disc-like slices, therebyproducing a plurality of silicon wafers as shown in FIG. 1( a) (step11). FIG. 1( b) shows the cross-sections of silicon wafers havingundergone the corresponding steps of FIG. 1( a).

According to the conventional method, since the peripheral edge of asliced wafer is rectangular, the peripheral edge is beveled because toprevent to develop notches and flaws there. However, according to theinventive method, a wafer is immersed in an acidic solution containinghydrofluoric acid (HF) and nitric acid (HNO₃) at a volume ratio of ⅛ to½ (HF/HNO₃) so that degraded superficial layers occurring on the frontand rear surfaces of the wafer as a result of machining can be removedand its edge surface can be beveled (step 12). Immersion of a wafer inan acidic solution containing hydrofluoric acid (HF) and nitric acid(HNO₃) at a volume ratio of ⅛ to ½ (HF/HNO₃) will enable the removal ofa part of degraded superficial layers occurring on the front and rearsurfaces of the wafer as a result of machining and beveling of its edgesurface to be accomplished simultaneously. By adjusting as appropriatethe blending ratio of hydrofluoric acid and nitric acid in the acidicsolution, it is possible to produce a solution which can bevel the edgeof a wafer by corrosion. Such a solution can effect beveling as observedafter mechanical beveling. Thus, the step 12 can substitute for themechanical beveling step conventionally employed in the processing of awafer, thereby reducing the chance of the intervention of alkalinecleaning which must be introduced subsequent to mechanical beveling. Ifthe volume ratio (HF/HNO₃) of hydrofluoric acid (HF) to nitric acid(HNO₃) is less than ½, that is, if the fraction of nitric acid isgreater than the specified amount, the etching rate will be higher thandesired, and thickness distribution on the surfaces will be degraded,and big warps will develop. Furthermore, it will be difficult to controlthe shape of the surface. On the contrary, if the volume ratio ofHF/HNO₃ is less than ⅛, that is, if the fraction of nitric acid is morethan the specified amount, the etching rate will be so low that too muchtime will be required for etching to be practicable. The volume ratio(HF/HNO₃) of hydrofluoric acid and nitric acid in the acidic solution ispreferably ⅕.

It is also possible by using an alkaline etching solution to bevel theedge of a wafer. However, etching a wafer having just undergonemechanical beveling which is usually contaminated with metal impuritiesusing an alkaline detergent such as a KOH or NaOH-based detergent willhave the risk of dispersing the metal impurities into the substrate ofthe wafer as well as over its surfaces. Accordingly, use of an acidicetching solution is more preferred. In terms of throughput, use of analkaline etching solution whereby etching occurs at a slower speed isundesirable.

The rate of etching effected by an acidic etching solution containinghydrofluoric acid and nitric acid is adjusted so that the total etchingrate for both surfaces of a wafer is in the range of 0.1 to 1.0 μm/sec.A preferable etching rate is 0.2 to 0.3 μm/sec.

Then, the front and rear surfaces of a wafer are mechanically abraded(lapping) so that the flatness of the surfaces of the wafer and theirparallelism can be improved (step 13). The thickness of the front andrear surface bitten away during lapping is preferably adjusted to be 15to 20 μm.

Having undergone lapping step 13, the wafer is cleaned by being immersedin an alkaline solution (step 14). The thickness of the front and rearsurface of the wafer eaten away during alkaline cleaning is preferablyadjusted to be less than 1 μm. The cleaning removes contaminantsresulting from machining. The alkaline solution preferably consists ofpotassium hydroxide.

Next, the wafer is alternately transferred between two groups of etchingtanks one of which contain acidic etching solutions and the otheralkaline etching solutions (step 16). During step 16, any degradedsuperficial layers which might occur on the front and rear surfaces ofthe wafer as a result of the lapping step 13 are completely eliminated.The etching step 16 includes acidic etching 16 a followed by alkalineetching 16 b. The alkaline etching solution has a concentration of 8mol/l or higher. If the concentration of the alkaline etching solutionis less than that lower limit, facets formed on the surfaces of thewafer will become large, pits having a size of several microns or lessand a depth of ten to several tens microns will develop, and thesurfaces of the wafer will be roughened. To compensate for those flaws,chemical and mechanical abrasion introduced in subsequent steps must beadjusted such that thicker superficial layers can be abraded thannormal. The concentration of the alkaline etching solution is preferablyadjusted to be 10 mol/l or higher. The rate of etching effected by anacidic etching solution is adjusted so that the total etching rate forboth surfaces of a wafer is 0.2 μm/sec or higher. A preferable etchingrate is 0.2 to 0.8 μm/sec.

It is necessary to insert a cleaning step 17 between the etching step 16a and the etching step 16 b. Insertion of a cleaning step 17 makes itpossible to remove acid or alkali adhered to a wafer resulting from thetreatment of a preceding step, thereby preventing thecross-contamination between the acidic etching tanks and the alkalineetching tanks, and minimizing the variation in composition of thesolutions stored in those tanks. Having undergone etching step 16, thewafer is cleaned of detergent adhered to its surfaces (step 18).

The wafer having undergone the cleaning step 18 will be subjected tosubsequent steps such as unilateral polishing, PCR (polishing cornerrounding) which mirror-polishes the beveled edge of the wafer, heating,unilateral polishing using a bilateral abrader, and cleaning, to beready to be further processed into a device.

INDUSTRIAL APPLICABILITY

The inventive method for processing a silicon wafer simplifies the stepsinvolved in the process, and reduces the risk of contamination due tometal impurities, which may result from cleaning subsequent to variousmachining processes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart representing the steps for processing a siliconwafer according to the inventive method.

FIG. 2 is a flowchart representing the steps for processing a siliconwafer according to a conventional method.

REFERENCE NUMERALS

1: Slicing step

12: Acidic solution immersion step

13: Lapping step

14: Alkaline cleaning step

16: Etching step

FIG. 1( a)

Single Crystal Ingot

-   11: Slicing-   12: Immersing wafer into acidic solution (HF/HNO₃=⅕)-   13: Lapping-   14: Alkaline cleaning-   16 a: Acidic etching-   17: Cleaning-   16 b: Alkaline etching    FIG. 2( a)    Single Crystal Ingot-   1: Slicing-   2: Cleaning with detergent-   3: Beveling-   4: Alkaline cleaning-   5: Lapping-   6: Alkaline cleaning-   7: Fine beveling-   8: Alkaline cleaning-   9: Acidic etching

1. A method for processing a silicon wafer comprising: a slicing step(11) in which a single crystal ingot is sliced into thin disc-likewafers; a lapping step (13) in which the surface of each wafer is lappedto be planar; an alkaline cleaning step (14) in which the wafer issubjected to alkaline cleaning to be removed of contaminants resultingfrom preceding machining; and an etching step (16) in which the wafer isalternately transferred between two groups of etching tanks one of whichcontain acidic etching solutions and the other alkaline etchingsolutions, wherein: a beveling step (12) is introduced between theslicing step (11) and the lapping step (13), in which the wafer isimmersed in an acidic solution containing hydrofluoric acid (HF) andnitric acid (HNO₃) at a volume ratio of ⅛ to ½ (HN/HNO₃) so thatdegraded superficial layers occurring on the front and rear surfaces ofthe wafer as a result of machining can be removed and the edge surfaceof the wafer can be beveled.
 2. A method of claim 1 wherein the etchingusing an acidic solution containing hydrofluoric acid and nitric acid isadjusted such that the total etching rate summed for both surfaces of awafer is in the range of 0.1 to 1.0 μm/sec.
 3. A method of claim 1wherein the etching step (16) includes acidic etching (16 a) followed byalkaline etching (16 b), the alkaline etching solution has aconcentration of 8 mol/l or higher, and the total etching rate summedfor both surfaces of a wafer during acidic etching is adjusted to be 0.2μm/sec or higher.